Report

4th year – Electrical Engineering Department GENERAL CHARACTERISTICS OF ANTENNAS Guillaume VILLEMAUD Antennas – G. Villemaud 0 Key Points We have seen that the antenna theory is based on the radiation produced by the sources (charges, currents) on the surface of a conductor. When we want to describe the operation of a particular antenna, some basic features common to all types of antennas are given: Input impedance Radiation pattern Gain Polarization Antennas – G. Villemaud 1 Example of Datasheet Access Point Antenna for WiFi systems Antennas – G. Villemaud 2 Example of Datasheet (2) Access Point Antenna for WiFi systems Specifications Electrical Gain Frequency Range VSWR Power Impedance Polarization Front to Back Ratio E-plane Beamwidth H-plane Beamwidth 8.0 dBi 2300-2500 MHz 1.5:1 10 watts 50 ohms Vertical >25dB 60°+-5° 80°+-5° Mechanical Depth Radiator Material Reflector Material Mounting Windload(fatal) Weight Cable Connector 1.6 inches (4.1 cm) Brass Brass Integrated 208kph 0.145 kg not supplied SMAfemale Antennas – G. Villemaud 3 Input impedance If we take the example of the open line, the distance between the arms causes a change in impedance. The wave is then reflected at the interface between the line and the antenna, with significant energy loss. The goal is then to return to a matched system. mismatch Zi ei Zr=Zc Zc Antennas – G. Villemaud 4 The antenna as a circuit Pa Pi generator Pe emitted power Pr Ze The antenna is a resonant (stationary wave) system, it must ensure that the impedance presented to the front line (its input impedance) is adapted to it. The line is in progressive wave, the power is fully transmitted to the antenna. The antenna is then used as an impedance transformer between the transmission line and free space. The radiated power depends on the accepted power and antenna losses. Antennas – G. Villemaud 5 Reflection Coefficient The quality of matching of an antenna is given by its characteristic impedance (usually 50 ohms), or by giving the reflection level. Ze R jX Reflection coefficient on power: S11 S11 2 Pr Pi is the reflection coefficient on voltage Input impedance deduced from reflection values: 1 S11 Ze Zc. 1 S11 Antennas – G. Villemaud 6 Expression in decibels Most of the time the values are expressed in decibels: S11 dB 20 log S11 return loss But we can also found the use of VSWR (Voltage Standing Wave Ratio): VSWR 1 S11 1 S11 Often expressed with the form: n:1 Antennas – G. Villemaud 7 Conversions VSWR Return Loss (dB) Reflected Power (%) Transmiss. Loss (dB) VSWR Return Loss (dB) 1.00 ∞ 0.000 0.000 1.38 15.9 2.55 0.112 1.01 46.1 0.005 0.0002 1.39 15.7 2.67 0.118 1.02 40.1 0.010 0.0005 1.40 15.55 2.78 0.122 1.03 36.6 0.022 0.0011 1.41 15.38 2.90 0.126 1.04 34.1 0.040 0.0018 1.42 15.2 3.03 0.132 1.05 32.3 0.060 0.0028 1.43 15.03 3.14 0.137 1.06 30.7 0.082 0.0039 1.44 14.88 3.28 0.142 1.07 29.4 0.116 0.0051 1.45 14.7 3.38 0.147 1.08 28.3 0.144 0.0066 1.46 14.6 3.50 0.152 1.09 27.3 0.184 0.0083 1.47 14.45 3.62 0.157 1.10 26.4 0.228 0.0100 1.48 14.3 3.74 0.164 1.11 25.6 0.276 0.0118 1.49 14.16 3.87 0.172 1.12 24.9 0.324 0.0139 1.50 14.0 4.00 0.18 1.13 24.3 0.375 0.0160 1.55 13.3 4.8 0.21 1.14 23.7 0.426 0.0185 1.60 12.6 5.5 0.24 1.15 23.1 0.488 0.0205 1.65 12.2 6.2 0.27 1.16 22.6 0.550 0.0235 1.70 11.7 6.8 0.31 Antennas – G. Villemaud 8 Reflected Power (%) Transmiss. Loss (dB) Radiation resistance Ze R jX Radiation resistance and loss resistance For purely metallic antennas, the loss resistance could be neglected. For a purely resitive antenna (accorded antenna), X=0 Antennas – G. Villemaud 9 Bandwidth There are many definitions of bandwidths. The most common is the bandwidth in impedance matching where the reflection coefficient of the antenna meets a certain level. Antennas – G. Villemaud 10 Relation to the impedance The complex impedance of an antenna varies with frequency. It corresponds to variations in current distribution on the surface. We try to match the Z(f) = R(f) + j X(f) operating frequency with a purely real résonance Serial impedance similar to série resonance that of system (usually 50 ohms). X(f) R(f) f Parallel mode resonance fondamental Antennas – G. Villemaud 11 Serial or parallel resonances The geometry of the antenna and its feeding mode affects the impedance. We usually try to place as close to resonance and cancel the imaginary part. Antenna Serial resonance Max of current at the generator Low impedance Parallel resonance Min of current at the generator High impedance Antennas – G. Villemaud 12 Examples of matching points Z, 60 W Matching zone Example of the dipole case n°1 40 20 Re(Z) 0 i I m(Z) -20 -40 0,2 Z, 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 f fr W 120 case n°2 v 100 80 I m(Z) 60 40 20 Re(Z) 0 -20 0,2 Z, 450 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 f fr W case n°3 350 250 Re(Z) 150 The choice of the feeding point can determine the bandwidth; I m(Z) 50 -50 -150 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 f fr Antennas – G. Villemaud 13 Mutual Coupling Two closely spaced antennas influence each other by a coupling of electromagnetic fields. This coupling must be taken into account because it changes the antenna characteristics (impedance and radiation). Rapid limitation of analytical models Electromagnetic modeling Antennas – G. Villemaud 14 Radiation characteristics To account for the performance of the antenna from the point of view of the radiated fields are used: The characteristic function (field pattern) The radiation pattern The directivity The gain The beamwidth The effective area And therefore to build the link between two antennas we will use the link budget (Friis’ formula) Antennas – G. Villemaud 15 Characteristic function The characteristic function is used to represent changes in the level of the radiated field in the far field zone as a function of the direction considered. Case of the Hertzian dipole: E ( ) I dl sin e j (t r ) 2r I : max. intensity 60 E ( ) I dl sin r 60 I dl E ( ) sin r j F ( ) Characteristic function of the hertzian dipole Antennas – G. Villemaud 16 Radiation Pattern Global definition: z r F ( , ) E , 60 I y Hertzian dipole x Vertical plane Antennas – G. Villemaud x Horizontal plane 17 Power Notion The total radiated power is equal to the flow of the Poynting vector through a closed surface surrounding the antenna. P P.dS sphere In farfield, it comes: 2 E P 2 Surface power density To represent this a normalized power is often used: Pn, P, Pmax Antennas – G. Villemaud 18 Solid Angle The power flow density can also be expressed in steric density according to the solid angle dW dW 1 dS sindd r2 2 15 2 Pe I F , dW W PeU , dW W U , 15I 2 F 2 , Watt / steradian Steric power density or radiation intensity Antennas – G. Villemaud 19 Radiation resistance When we link between the radiated power and the power dissipated by a load, we can determine the radiation resistance from the characteristic function. 2 15 2 Pe I F , dW W F , dW Rr 30 2 W Antennas – G. Villemaud 20 Antenna Directivity Pe is the total radiated power, it is said that the antenna is isotropic when the steric density in any given direction is expressed as: U , Pe 4 We call directivity the relationship between power density created in a given direction and the power density of an isotropic antenna. U , D , Pe 4 Antennas – G. Villemaud 21 Meaning of the directivity F 0,0 D0,0 1 F 2, dW 4 W 2 For isotropic antenna, D=1 whatever the direction Antennas – G. Villemaud 22 Antenna Gain The gain is defined in the same way as the directivity, but taking into account of the power supplied to the antenna: U , G , Pf 4 This gain is sometimes called actual or realized gain as opposed to intrinsic gain not taking into account all the losses of the antenna (without loss of mismatching). Gintrinsic If there is no loss, the gain is equal to the directivity Grealized 1 S11 2 F 20,0 G0,0 4 2 , dW F W Antennas – G. Villemaud 23 Relation to the resistance Starting from: F 20,0 G0,0 4 2 , dW F W We can give a simple formula to calculate the gain function form the radiation resistance : 120F G o,o 2 Rr Still in the no matching loss hypothesis Antennas – G. Villemaud 24 Radiation pattern teminology Half-power beamwidth(-3dB) Axis of the main lobe Zero of radiation 1 Secondary lobes (sidelobes) 0,8 0,6 0,4 Antennas – G. Villemaud 25 Types of representation There are a multitude of ways to represent the radiation of an antenna: field pattern, power pattern, gain, directivity, polar or Cartesian, linear or decibels, 2D or 3D Antennas – G. Villemaud 26 Example of microwave bridge Radiation pattern Linear radiation pattern (P/Pmax) 1 0 0.8 -20 0.6 P G (dBi) 20 -40 0.4 -60 0.2 -80 -200 -100 0 angle (°) 100 200 0 -200 -100 90 120 120 30 180 330 240 200 60 150 0 210 100 90 60 150 0 angle (°) 30 180 0 210 300 270 330 240 300 270 Antennas – G. Villemaud 27 Reference planes Z Excited mode: :TM10 Mode excité H E H H plane Plan H E plane Plan E Radiating element Courants d e surface liés la cross-polarization: po larisation croisée: Surface currents linked to àthe Jx Jy Courants surface liés à la polarisation principale:Jx Surfacedecurrents linked to the main polarization: Jy Antennas – G. Villemaud 28 Measurement methods Vector Network ananlyzer Impedance matching measurements Antenna under test TA Horn RF out motion motion motion Motion control Directional coupler VNA Radiation measurements Computer Antennas – G. Villemaud 29 Measurement chambers Antennas – G. Villemaud 30 Measurement chambers Antennas – G. Villemaud 31 EIRP When an antenna produces a radiated power Pe, the power density created in a given direction is the product of the gain in this direction by the power. The Equivalent Isotropic Radiated Power is: EIRP=Pe.Ge This value is particularly usefull for standard’s definition. Antennas – G. Villemaud 32 Effective area An antenna illuminated by a plane wave of power density Ps, we call effective area of the antenna quantity: load S, Pd Ps From the gain : G , Ps Pf 4r Antennas – G. Villemaud 33 Effective area and gain If we build a transmission between two antennas: Pf Pd load antenna 1 antenna 2 Pd S2 G1 S1 G2 Pf 4r 2 4r 2 Reciprocity : Then: G1 G2 S1 S2 If we take the hertzian dipole as example, it comes: Antennas – G. Villemaud S, GA, 4 2 34 Link Budget Friis’ formula or link budget is used to calculate the power available at the receiver load depending on the power supplied to the emitting antenna. We know Pr S2 G1 Pe 4r 2 S2 G2 4 2 or 2 Pr Ge, Gr, Pe 4r Antennas – G. Villemaud 35 More detailed budget link The previous formula assumes matched loads and the same antenna polarization. Otherwise, a more complete budget can be made: F G , F G , P . Pd 4r 2 A e B r f p It takes into account the impedance matching of antennas, their gains in the direction of communication and polarization efficiency. Antennas – G. Villemaud 36 Decibel expressions An expression given in decibels is always relative, so a value relative to a multiplication or a division in the linear domain. As we are expressing power values, we always use 10log (ratio). This is still consistent with calculations with the field values in 20log. To express absolute values like power level, we use a reference value, which could be 1 mW (dBm) or 1 W (dBW). Directivities or gains are expressed in dBi (relative to isotropic) or dBd (relative to dipole). Attenuation or amplification terms are expressed in pure dB. Antennas – G. Villemaud 37